The present invention relates to a signal processing system for a video cassette recorder (VCR) yielding improved video quality. Current home VCRs record video information onto video tape cassettes in one of several formats. The well-known VHS system produces degraded picture quality chiefly because there is insufficient horizontal resolution. An enhanced VHS system, popularly called Super VHS (S-VHS), produces enhanced picture quality by recording a full-bandwidth video signal on the video tape cassette, thus yielding improved picture quality. Such a system requires higher quality tape in the cassette and higher quality recording and playback mechanisms and circuitry. However, the S-VHS system is not backward compatible with standard VHS VCRs. That is, although a S-VHS VCR can play back cassettes recorded on either S-VHS or standard VHS VCRs, a standard VHS VCR cannot play back cassettes recorded on S-VHS VCRs.
It has long been a goal of video engineers to increase the amount of information transmitted through a given narrowband channel, such as NTSC, which is limited to 4.2 MHz of bandwidth. Because the frame and line rates are usually fixed, restricting the bandwidth translates into restricting the horizontal resolution. In some cases, the bandwidth of the channel is limited to 3 MHz or even 2.5 MHz resulting in an image with insufficient horizontal resolution.
It has been long recognized that the video spectrum has holes in which the signal energy is very small. The NTSC color system represents a system which uses one of these holes to carry the color information. In the NTSC system, the color information is recorded on a color subcarrier whose frequency is very carefully selected so that a minimum disturbance occurs when a color signal is displayed on a black and white receiver. Specifically, the frequency of the color subcarrier is interleaved horizontally,vertically, and temporally to minimize the crosstalk between the luminance and chrominance components of the composite video signal.
It was recognized at around the time of the adoption of the NTSC system that such frequency holes could also be used to transmit additional horizontal information to increase the horizontal resolution of the reproduced image. In these systems, the high frequency horizontal information was interleaved with the low frequency horizontal information in a similar manner as the chrominance information is in the NTSC color system. An article titled "REDUCTION OF TELEVISION BANDWIDTH BY FREQUENCY INTERLACE" by Howson and Bell contains a description of such a system which operated in the analog domain. This system, however, could not accurately reproduce the full-bandwidth image in its original form because it was unable to completely remove the artifacts resulting from the frequency interleaving, which manifested themselves as annoying dot crawl patterns.
Sampled data digital video signal processing techniques were later developed using subNyquist sampling (sometimes termed subsampling) that remove the artifacts resulting from the frequency interleaving better than can be done in the analog regime. These techniques involve replacing every odd sample in a first video line with a zero-valued sample, and then on the next line, replacing every even sample with a zero-valued is sample. On alternate frames, the patterns are reversed.
German Patent Application 82100286.2 entitled "Verfahren zum Ubertagen von Fernsehsignalen uber einen genonrten bandbreitebegrenzten Ubertragunskanal und Anordnung zum Durchfuhren des Verfahrens," filed Jan. 1, 1982 by Professor Wendland et al, describes principles of offset sub-sampling and bandwidth compression as applied to advanced television systems. This patent also describes techniques for implementing television systems in accordance with the described principles.
Theoretically, the Howson and Bell frequency folding techniques and the subNyquist sampling technique are equivalent. But, although theoretically equivalent, the later sampled data digital systems provide improved reconstruction of the received image because of the existence of line and frame combing techniques, which had not been developed at the time of the Howson and Bell system. The sub-Nyquist sampling techniques, however, were developed for totally sampled data digital systems as data reduction techniques. Signals generated by these systems were not intended to be passed through a narrowband analog channel.
In an article "DEVELOPMENT OF HDTV RECEIVING EQUIPMENT BASED ON BAND COMPRESSION TECHNIQUE (MUSE)", by Kojima et al. in IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 4, November 1986, pp. 759-768, another data compression scheme is described which achieves bandwidth compression by sampling each pixel once every other frame. This scheme works well for nonmoving images. For moving images, a motion vector is developed, and the actual rate of sampling of each pixel is adaptively varied in response to the motion vector so that a sample of the pixel is transmitted every other frame on the average, but more often when that pixel is representing a moving image.
U.S. Pat. No. 4,831,463, issued May 16, 1989 to Faroudja, describes apparatus for processing a video signal having a predetermined bandwidth in order to pass the video information through a limited bandwidth channel, such as magnetic tape. In the apparatus described in this patent, a video signal preprocessor includes a comb filter to produce a spectral hole, such as described above, between spectrally active areas in the video signal spectrum. The combed video signal is mixed with a folding carrier signal located at a frequency just above the uppermost video signal frequency to generate an upper sideband that repeats the video signal spectrum without spectrum reversal, but with frequency translation, and a lower sideband that repeats the video signal spectrum with spectrum reversal. A lowpass filter then filters the mixer result so that its bandwidth is about one-half the bandwidth of the original video signal. The mixer and lowpass filter comprise a folding circuit of the type previously built by Howson and Bell. This folding circuit, as employed by Faroudja, in effect folds the high frequency video components of the video signal about a predetermined folding frequency, selected so that the folded high frequency component is placed in the spectral hole previously made in the video signal. The resulting signal may then be transmitted through the limited bandwidth channel.
Faroudja's U.S. Pat. No. 4,831,463 further describes a post processor which receives a folded signal from the limited bandwidth channel. The post processor includes an unfolding circuit which unfolds the received signal about a predetermined unfolding frequency. A comb filter then processes the unfolded signal to remove the alias components resulting from the unfolding process. The signal produced by this comb filter closely approximates the original video signal in terms of the bandwidth and information content.
Both the Howson article and the Faroudja patent describe folding systems which, if incorporated into an improved VCR, would not produce cassettes which would be able to be played back on present VCRs without introducing artifacts that are quite visible and therefore unacceptable in a commercial sense. This is primarily due to the magnitude of the folded high frequency component present within the low frequency component, on the previously recorded cassette. The magnitude of the folded high frequency component is sufficiently high as to introduce intolerable artifacts into an image produced from a video signal from which the folded high frequency component were not properly removed.
The Faroudja patent does not include any discussion of compatibility with pre-existing record media and apparatus, other than mention that it is one object of that invention. There is no teaching of any apparatus or process for achieving backward compatibility.
Howson and Bell did not concern themselves with backward compatibility, and the apparatus of the type they built includes a pre-emphasis filter, which boosts the high frequency components of the luminance signal in order to minimize the effect of crosstalk from the low frequency luminance components during the transmission of the folded signal through the channel. If a video cassette recorded by a VHS VCR modified to include the system described by Howson and Bell were played back on a standard VHS VCR, the interference of the pre-emphasized high frequency components which would not be removed would produce an even more objectionable image than the produced by the Faroudja system.
It is desirable that an improved video recording system be able to record wider bandwidth video signals on a standard quality cassette than those recorded by present VCRs, but maintain backward compatibility with present VCRs, and not require especially high quality magnetic tape or record and playback mechanisms. That is, it is desirable that standard quality cassettes which are recorded using the improved system be able to be played back, without noticeable visual artifacts, on present VCRs (even if the present VCR may not be able to reproduce the full-bandwidth signal recorded on such a cassette).